1. Field
The present invention is directed on a radiofrequency plasma reactor, a vacuum treatment system comprising such reactor and on a method for manufacturing a vacuum process treated substrate.
2. Description of Related Art
Plasma is commonly understood as an ionized gas comprising ions and electrons. The plasma is electrically conductive and strongly responds to electromagnetic fields. Plasma is often generated by an electric discharge in a vacuum wherein a working gas, normally a noble gas like argon, is introduced and ionized in an electromagnetic field to result in ions and electrons. Additionally or instead of such working gas it is common praxis, depending upon the treatment of a substrate to be performed with the help of such plasma, to introduce a reactive gas into the plasma discharge space which is activated in the electromagnetic field. The activated reactive gas directly interacts with a surface of a substrate to be treated or forms a reaction product in the plasma discharge, which latter interacts with the addressed surface. There are a large number of reactive gases which are exploited for vacuum plasma treatment of substrate surfaces. We refer to a few examples as hydrogen, nitrogen, silane, SF6. The reactive gases are selected according to the desired treatment of substrate surface. Such a treatment may be e.g. deposition of a solid material layer upon the substrate surface, surface treatment of the addressed surface as e.g. by oxidizing, etching, thereby unselectively or selectively removing material from the substrate surface, or heating the substrate. The present invention is thereby especially directed on such a treatment of large substrate surfaces, which have an extent of at least 1 m2. Thereby, we speak of “a substrate” thereby understanding either one distinct substrate or two or more distinct substrates which are simultaneously treated and which concomitantly have a surface to be treated of the addressed at least 1 m2.
The treatment of large substrate surfaces, especially of single piece substrates, is required e.g. for manufacturing substrates in the frame of TFT panel manufacturing or for manufacturing photovoltaic panels.
Thereby, it is known in this art to perform such a treatment in a vacuum plasma which is operated by an electrical supply generating plasma discharge by means of electric power which at least comprises a significant Rf component. We generically address such a plasma as a radiofrequency plasma. Generically, the electric supply for a radiofrequency plasma may comprise additionally to radiofrequency components lower-frequency components down to DC and may be applied as a pulsating signal or as a more continuous wave signal.
We thereby understand in the frame of the present application under “radiofrequency” frequencies which are at least 5 MHz.
In the art of vacuum plasma processing substrates so-called parallel plate reactors are widely known. Such parallel plate reactors are thereby also widely known conceived as Rf plasma reactors. As shown in FIG. 1 such a known Rf plasma reactor comprises a vacuum recipient 1, wherein there is provided a first electrode arrangement 3 and a second electrode arrangement 5. The first electrode arrangement 3 defines for a first electrode surface 3E, whereas the second electrode arrangement 5 defines for a second electrode surface 5E. The first and second electrode surfaces 3E and 5E are distant from each others and face each others. They define concomitantly the plasma discharge space PD. Depending upon the specific purpose of such radiofrequency plasma reactor with respect to substrate surface treatment the two electrode surfaces 3E and 5E define for a spacing distance d along the addressed surfaces which is constant all along the surface area along which the electrode surfaces 3E, 5E face each other or define for a distance d which may vary along the addressed extent. In the case of constant distance d along the electrode surface area along which the electrode surfaces face each other it is known as shown in FIG. 1 to tailor the electrode surfaces 3E, 5E to be plane.
Both electrode arrangements 3 and 5 are operationally connected to pole I, II respectively of an electric Rf supply unit 7. Thereby, different possibilities are known to operate the Rf supply unit 7 with respect to an electric reference potential Φo, i.e. with respect to ground potential. As shown in FIG. 1 at “a” one possibility is to operate the Rf supply unit 7 in an electrically floating manner with respect to the recipient 1. A second possibility shown in dash line in FIG. 1 and referenced by “b” is to operate the recipient at reference potential and additionally to operate the Rf supply unit 7 on reference potential so as to apply either symmetrically or asymmetrically Rf power to the respective electrode arrangements 3 and 5. A further possibility also shown in FIG. 1 and addressed by reference “c” is to operate one of the electrode arrangements 3 or 5 on reference potential as well. In such a reactor a substrate is provided either deposited on one of the electrode surfaces as shown in FIG. 1 at reference No. 8, thereby and if one of the electrode arrangements is operated on reference potential customarily on the electrode surface of that electrode arrangement. Alternatively and as shown in FIG. 1 in dash line at 8a the substrate may be operated in the plasma discharge space remote from both electrode surfaces, thereby electrically floating or tightened on an electric bias potential as shown in dash line in FIG. 1 at 10, which bias may be DC and/or AC. When we address the substrate 8a being operated electrically floatingly we mean that the electric potential whereupon the substrate 8a resides during treatment operation establishes for electric potential distribution within the plasma discharge space PD.
For treating large substrates as addressed above obviously the electrode arrangement and thereby the electrode surfaces must be tailored with respective extents which are at least similar with the extent of the substrate, customarily even larger. Thereby and dependent on one hand from the substrate extent and on the other hand on the frequency of the Rf power operating the plasma discharge, the dimension of the electrode surfaces on one hand and the wavelength of the Rf signal become of comparable extent.
Thereby, standing waves phenomena are likely to occur in the plasma discharge space resulting in an inhomogeneous distribution of plasma treatment effect along the surface of the substrate.
From the U.S. Pat. No. 6,228,438 it is known to counteract the addressed phenomenon by providing at one of the electrode arrangements 3 or 5 according to FIG. 1 a dielectric spacing 12. The dielectric spacing 12 of a dielectric solid material, e.g. a ceramic material, and/or realized by a void compartment resides on a metal Rf supply electrode 14 which is operationally connectable to the first pole I of the Rf power supply unit 7. The dielectric spacing 12 forms between the electrode surface 3E and a metal surface 15 of the supply electrode 14 a capacitive arrangement. As exemplified in FIG. 1 the spacing dc measured perpendicularly to the respective surface area of the supply electrode 14 varies along the extent of the addressed surface 15 and thus along the extent of the electrode surface 3E. Thereby, there is established a distributed capacitive Rf coupling between the supply electrode 14 and the electrode surface 3E with a coupling capacitance value which varies along the extent of the electrode surface 3E. The result is that the occurrence of standing waves is substantially prevented for the treatment of large substrates at Rf frequencies above 10 MHz.
With an eye on FIG. 1 please note that it is a schematic representation of the Rf plasma reactor wherein additional members as e.g. vacuum pumping units, gas supply members, loadlocks, etc. have not been shown.
From the EP 0 953 204 it is further known to conceive one of the electrode surfaces of a Rf plasma reactor as generically shown in FIG. 1 by a pattern of mutually electrically isolated metal electrode members which are in groups operated on distinct electric potentials.
Turning back to the approach according to the U.S. Pat. No. 6,228,438 the dielectric spacing 12 is commonly and at least to a predominant part made of a ceramic material. Thereby, the addressed ceramic material of the dielectric spacing is exposed during treatment of the substrate to high thermical loading and, depending upon the selected treatment, to the effect of aggressive reactive gases or reaction products thereof. This is at least one important reason to select for conceiving the dielectric spacing a ceramic material. Such ceramic material is difficult and expensive in manufacturing and shaping and is brittle, a drawback e.g. for assembly, operation and maintenance. Providing cooling systems, as e.g. a system of piping for a cooling medium within such ceramic material dielectric spacing, is extremely difficult to realize and thus costly.